We have produced crystals of the extracellular domain of the high affinity IgE receptor from mast cells. This receptor provides a crucial link between soluble antibody and cell-based mechanisms of immunity. The receptor-IgE interaction is the key molecular recognition step associated with immediate-type hypersensitivities such as allergic and anaphylactic shock reactions. The crystals diffract weakly on laboratory sources and we are requesting time to collect high resolution data at SSRL.